A bomb went off in a building in a downtown Warsaw (search) business district and injured one person, punching a hole in the building wall and shattering windows, police said.

Authorities do not suspect terrorism, but instead believe the bomb, hidden in a briefcase, was planted by someone with a grudge, police spokesman Mariusz Sokolowski said. The bomb detonated at about 1 p.m. in a hallway of the building, which is owned by Kolmex (search), a Polish company that provides services and products to railway companies.

Kolmex sublets some offices to other companies, including Sony Corp.

"We suspect that it wasn't terrorism, but rather a settling of accounts," Sokolowski told The Associated Press.

Poland, a staunch supporter of the U.S.-war on Iraq and commander of the multinational force there, was threatened with reprisal attacks in July in an online statement from a group calling itself Tawhid Islamic Group (search).

Piotr Cichowlas, 27, a cook in a bar across the street, said he had just pulled up for work when he heard "a very loud explosion."

"I looked up and I saw black smoke and glass falling from the window," he said. "There were just a few people in front of the building and they ran to get away from the falling glass, but there was no big panic."

People were evacuated from the building and from two buildings across the street, and the injured man was taken to a hospital, authorities said.

The only signs of the blast were three smashed windows on the top floor.